The three-dimensional fiber path pattern in the mammalian heart, at the microscopic level of resolution, is currently unknown. However, the macroscopic pattern is known to be similar in different mammals. Therefore, an analysis of the microscopic fiber path pattern in the mouse will significantly increase our understanding of the pattern in the human heart. As a first step, the fiber path pattern in the adult mouse will be reconstructed from serial sections, using digitized coordinates and computer techniques. Then progressively younger prenatal mouse hearts will be examined to determine when an organized pattern emerges. This could be helpful in understanding the development of congenital cardiac lesions. Then, a sampling technique based on careful analysis of the extensive body of data obtained in the first two steps will be devised. The final step will be to use this sampling technique to reconstruct the fiber pattern in the postnatal human heart, both normal and congentially abnormal. The work is expected to contribute to a better understanding of the relations between the fiber geometry of the heart and its biophysical properties, such as contractility and conductivity, as well as to an understanding of the formation of certain congenital abnormalities of the human heart.